In the process described in application Ser. No. 188,254, a sulfur dioxide-containing gaseous stream is contacted with a solution containing magnesium hydroxide in a scrubbing unit with a portion of the spent scrubbing medium oxidized and the oxidized product treated with a magnesium-containing lime slurry to obtain regenerated magnesium hydroxide for recycle to the scrubbing unit. In that process, scrubber liquor sulfites are oxidized to sulfates with the addition of a magnesium-containing lime to the oxidized scrubber liquor to precipitate magnesium hydroxide and calcium sulfate, and the calcium sulfate or gypsum is separated from the magnesium hydroxide. The magnesium hydroxide is then recycled t the scrubbing unit since it is the alkali required by the scrubbing system.
In the method described in application Ser. No. 395,667, calcium sulfite present in an aqueous sludge from a flue gas desulfurization (FGD) system is separated as a sludge containing at least 20 weight percent solids from the scrubbing slurry, contacted with acid to dissolve the calcium sulfite, and oxidation of the calcium sulfite effected to form a calcium sulfate precipitate. The calcium sulfate precipitate is then separated from the aqueous media as high quality gypsum.
In flue gas desulfurization systems where an aqueous slurry is used that is formed from calcium hydroxide and a predetermined amount of magnesium hydroxide, and where a magnesium containing lime is used to form the slurry, magnesium oxide dissolves and the magnesium, in the form of Mg.sup.++ ion, accumulates in the water contained in the system. Such magnesium is eventually discarded along with the calcium sulfite sludge that is produced in the scrubbing process. Since magnesium oxide is quite valuable as compared to lime, any recovery of the magnesium from the flue gas desulfurization sludge would be beneficial. Recovery of such magnesium will also reduce runoff of soluble magnesium salts from landfills that contain calcium sulfite produced in a scrubbing process. Recovery of magnesium will also make the use of magnesium-containing lime more attractive in FGD systems because a valuable by-product can easily be recovered.